Introducing Atmospheric River Index

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Discussion: The rapid increase of the climate research community to introduce new identifiers to help explain the increased incidence of dramatic weather and climate events in which impacts are becoming more extreme due to shifting and increasing population leads to another climate index: atmospheric river index.

How it works: “The intention of the scale is to describe a range of scenarios that can prove beneficial or hazardous based on the strength of atmospheric rivers” as determined by the amount of water vapor within the atmospheric river. The index ranks rivers from 1 to 5 with 1 the weakest and 5 the strongest index. The index is also based on duration lowering by 1 category when less than 24 hours and rising 1 category when present more than 48 hours.

Here is the scale…

  • AR Cat 1 (Weak): Primarily beneficial. For example, a Feb. 2, 2017 AR hit California, lasted 24 hours at the coast, and produced modest rainfall.
  • AR Cat 2 (Moderate): Mostly beneficial, but also somewhat hazardous. An atmospheric river on Nov. 19-20, 2016 hit Northern California, lasted 42 hours at the coast, and produced several inches of rain that helped replenish low reservoirs after a drought.
  • AR Cat 3 (Strong): Balance of beneficial and hazardous. An atmospheric river on Oct. 14-15, 2016 lasted 36 hours at the coast, produced 5-10 inches of rain that helped refill reservoirs after a drought, but also caused some rivers to rise to just below flood stage.
  • AR Cat 4 (Extreme): Mostly hazardous, but also beneficial. For example, an atmospheric river on Jan. 8-9, 2017 that persisted for 36 hours produced up to 14 inches of rain in the Sierra Nevada and caused at least a dozen rivers to reach flood stage.
  • AR Cat 5 (Exceptional): Primarily hazardous. For example, a Dec. 29 1996 to Jan. 2, 1997 atmospheric river lasted over 100 hours at the Central California coast. The associated heavy precipitation and runoff caused more than $1 billion in damages.

Comment: Climate Impact Company issues statements on presence, intensity and forecasts of the Madden Julian oscillation (MJO) almost daily as to the influence of this transient intra-seasonal tropical phenomena. The MJO (discovered by Madden and Julian in 1972) is an east-shifting large zone of tropical convection which when tapped by the pole ward jet streams causes atmospheric rivers of water vapor that lead to mid-latitude weather extremes and storms. For the most part, the AR index better quantifies the MJO influence on West Coast climate.